1. Field of the Invention
The present invention relates to a method for recycling a battery pack accommodating a sealed-type nickel-metal hydride secondary battery by collecting the battery pack from the market when it is judged as being degraded and renewing it.
2. Description of the Related Art
A nickel-metal hydride secondary battery of the “liquid-starved type”, which is an example of a sealed type alkaline secondary battery, has the advantages of being maintenance free and having a long charge/discharge cycle life. According to these advantages, there has been a high demand for the liquid-starved type nickel-metal hydride secondary battery in the market for pure electric vehicles (PEVs), hybrid electric vehicles (HEVs), etc., which need a secondary battery having an extremely long lifetime.
However, the lifetime of the liquid-starved type nickel-metal hydride secondary battery is over when an electrolyte inside the battery decreases to bring about a condition of liquid exhaustion, thereby increasing an internal resistance of the battery. When the battery is used in a vehicle such as a PEV or HEV, this may lead to the following problems. If the lifetime of the battery is shorter than that of the vehicle, the lifetime of the vehicle is limited by the lifetime of the battery. In addition, the cost required for replacing the battery pack with a new one is high.
On this account, for the purpose of extending the lifetime of the battery, JP 2000-215882 A, for example, discloses a method in which a screw-type inlet that can be opened and closed is provided in a battery case, and when the internal resistance of the battery becomes greater than 1.5 times the initial internal resistance, an electrolyte is added via this inlet until the internal resistance becomes not more than 1.5 times the initial internal resistance.
However, if the inlet that can be opened and closed is provided in the battery case, there is a possibility that an alkaline electrolyte having upward wicking characteristics along the inner wall of the battery case leaks from the inlet, and it is difficult to prevent such leak from occurring. Moreover, since the electrolyte needs to be added via the inlet by the user himself, the advantage of the sealed type alkaline secondary battery as being maintenance free cannot be taken.